A quantitative systematization of hydrogen-induced changes in Debye temperatures of metals

Nuclear resonance scattering of γ rays has recently been used to obtain information on the Debye temperatures θ_D, of metal hydrides. In particular the contribution of nickel to the θ_D of Mg₂Ni, LaNi₅ and their hydrides was estimated. It was assumed that the different behaviour of θ_D for nickel in the two cases (i.e. an increase upon hydrogenation in Mg₂Ni and no change in LaNi₅) could be related to the different stabilities of the two hydrides. Consequently an empirical relationship connecting the Debye temperatures θ_M and θ_H of a metal and its hydride respectively with the heat ΔH_f of formation of the hydride was derived from experimental data reported by various investigators. It appeared, however, that two ambiguous calculating procedures were used to derive the θ_H values. One of these procedures was used consistently in the present work for all the metal-hydrogen systems and the original phenomenological rule was changed accordingly. The qualitative theoretical considerations preceding the derivation of the empirical relation are briefly presented. Some experimental data recently reported in the literature are discussed in terms of the proposed empirical rule. In particular the obvious exceptions associated with the group IVa metal hydrides TiH_x and ZrH_x are attributed to the Jahn-Teller effect in these systems.